Contamination of roadside soils by metals linked to catalytic converters in Rio De Janeiro, Brazil

The contamination of soils due to traffic-related activities has been an ever-growing concern since the mid-20th century. This global issue impacts both rural soils often used for agriculture and grazing, and urban soils in densely populated urban areas. Therefore, this work aimed to evaluate the distribution of catalytic converters-related elements (Pd, Pt, Rh, Mo, and Ni) in roadside soils from the state of Rio de Janeiro, Brazil. Four highways, Linha Vermelha, Avenida Brasil, Via Dutra, and BR-465, were selected according to their daily traffic volume and urban/non-urban settings. Linha Vermelha and Avenida Brasil cross areas with high population density, while Via Dutra and BR-465 intersect non-urban locations. In addition to several physicochemical parameters, soil concentrations of three platinum group elements (Pd, Pt, and Rh), Mo and Ni were determined in samples collected at different distances from each highway (1, 3, 5, 10, and 15 m). While the results for Mo, Ni, Pt, and Rh are within reference levels in all highways, alarming concentrations of Pd were found, especially in soils bordering urbanized areas – note that Pd emissions to roadside soils are unique to automobile catalytic converters. Hence, as per the calculated geoaccumulation indexes, soils adjacent to Linha Vermelha and Avenida Brasil are either highly polluted or extremely polluted by Pd at all sampling points (averaging concentrations of 376 and 112 μg kg−1, respectively). The high traffic volume, frequent traffic congestion, and incidence of anthropogenic processes are the most probable roots for greater metal pollution in soils neighboring the selected urban highways. Contrariwise, soil metal levels in the non-urban areas are mainly dependent on traffic emissions and thereby in agreement with the pertaining literature, as concentrations decline with increasing distances from the road. Moreover, in comparison with the soils adjoining the urban highways, the acidic pH, reduced exchange capacity, and high organic matter content, suggest an insignificant contribution from other anthropic activities at Via Dutra and BR-465.